This invention relates to sewing apparatus, and in particular to a method and apparatus for detecting an aberrational stitch in real time.
A stitch is formed on a chain stitch sewing machine from two threads, a needle thread and a looper thread. As is well known, for and given type of machine, the threads stop and start in repeatable patterns during the generation of each stitch. These patterns are, essentially, independent of the speed of the sewing machine.
When a stitch is malformed, it is important to learn of that malformation and, in most instances, stop the sewing apparatus to correct for the malformed stitch. Generally, a malformed stitch is a skipped stitch, although other problems, such as a stitch that is too tight or broken thread must also be immediately detectable and the apparatus stopped as quickly as possible.
Various devices have been developed to detect aberrational stitches. For example, U.S. Pat. No. 5,383,417 monitors the stitching process to detect a skipped stitch. In one version of the apparatus, an optical detector is used to sense thread movement and break the beam of the optical sensor at the same point in time in each acceptable stitch. When a skipped stitch occurs, however, the beam is not broken, or is broken at a different time, and that data is used to stop the sewing machine. However, given the nature of the machine, detection is not in real time, and stopping occurs only after a lag.
Various apparatus has been developed to monitor breaks in the sewing machine thread. U.S. Pat. Nos. 4,841,890; 5,199,365 and 5,359,949 are examples of thread break monitors. Similarly, U.S. Pat. No. 4,805,544 uses an optical detector to determine when a bobbin has run out of thread.
U.S. Pat. No. 4,192,243 discloses a system to determine whether the stitching is too loose or too tight. It compares the number of stitches to the consumption of thread by the sewing machine. If the stitch count to thread usage is low, the stitches are considered to be too loose and an alarm is generated. If the stitch count to thread use is high, the stitches are considered to be too tight and an alarm is also generated. Similarly, broken thread is also detected.